Systems and methods are known in the art to detect the presence of a person or object at the entrance of an automatic door such as an elevator door or a pedestrian sliding door. These systems usually include infrared (IR) and/or microwave detector above the door which registers the increase of frequency of the emitted beam returning to the detector as a person moves towards the entrance, and more generally sense the motion coming from the objects to be detected.
It is also known in the art to use such systems on both sides of a pivoting door for example so as to prevent the door from colliding a person arriving on one side thereof while the door is caused to open by another person on the other side.
Radar technologies have also been proposed as an alternative to microwave technologies in such systems.
However, systems and methods known in the art suffer from the following drawbacks:                radar and passive IR technologies can only be used in application where the target to detect is moving;        especially in the case of IR-based systems, the environment has to be controlled, such systems being sensitive to rain, dust and light conditions;        IR sensors are also sensitive to the surroundings, thereby being prone to false alarms. They cannot operate when the door enters the line of sight of the detector, and cannot be active when the door is moving; and        IR sensors, both active and passive, are not very accurate and therefore have to detect relatively long distance from the floor.        
A typical solution in the art to this last specific problem, which is more common in elevator door applications, is to provide IR crossing beams in the door entrance. This however has the above-mentioned limitation of IR sensors, such as sensitivity to dirt, and also, since the resulted beams are very thin, they do not cover the whole opening. Furthermore, IR crossing beams are only applicable with sliding doors.
Ultrasound detecting systems for a door which are described in United States Patent Application Publ. No. US-2010-0319256-A1 titled “Presence Detector for a Door Assembly” and filed by Agam et al. on Aug. 23, 2010 aim at solving the above-described drawbacks.
Systems proposed by Agam et al. include a transducer assembly mounted to the door or to the door frame that emits ultrasound detecting beams towards the door entrance and triggers a detecting signal whenever a body intersects the detecting beams.
The same document discloses systems that cope for the movements of the door during operation. These adaptive systems suffer to some drawbacks. First, they do not allow determining the instant position of the door and are operable only with door assembly equipped with a further door position sensor. Second, the adaptive detection system described by Agam et al. are secured to the door and as thus, are movable in unison therewith. This limits the coverage zone of the system and renders the system less reliable.
An improved adaptive ultrasound system for a door assembly is therefore desired.